Munition Comprising a Body, an Explosive Charge and Wedging Means Between the Body and the Explosive Charge

ABSTRACT

A munition, such as a missile, a rocket or a projectile, includes a body forming an internal volume and an explosive charge confined in the internal volume. It applies notably, but not exclusively, to munitions of which the explosive charge comprises coated explosive molecules in a polymer binder. The explosive charge substantially conforms to the shape of an internal longitudinal surface of the internal volume, the internal surface being substantially of revolution about an axis and including protruding portions and/or cavities making it possible to keep the explosive charge wedged in the body.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to foreign French patent application No. FR 0905869, filed on Dec. 4, 2009, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a munition, such as a missile, a rocket or a projectile comprising a body forming a casing and an explosive charge confined in the casing. It applies notably, but not exclusively, to munitions of which the explosive charge comprises explosive molecules coated with a polymer binder.

BACKGROUND

Currently, explosive munitions are usually charged with an explosive of the type called poured-molten explosive, that is to say an explosive that is poured in the liquid state into the body of the munition and that solidifies while returning to a temperature below its melting point. However, new, safety-related requirements are leading to changing the type of explosive. These requirements are notably imposed by standards relating to munitions with attenuated risks, which aim to make munitions less sensitive to accidental and terrorist attacks. A new type of explosive has therefore been developed. This type of explosive comprises explosive molecules coated in a polymer binder, for example a cyclonite coated in a polybutadiene binder. The explosives of this type are called composite explosives. They make it possible to comply more easily with the insensitivity requirements. However, the physical properties of composite explosives raise new problems. A first problem is due to their coefficient of thermal expansion which is relatively high compared with that of poured-molten explosives. Since munitions are subjected to considerable variations in temperature, the explosive charge sustains expansions which make it not possible to bond it over the whole internal surface of the body of the munition. These new charges are therefore usually free or virtually free in the body of the munition, so that they are not in a permanent constricted state and they do not crack over time. Composite explosives are, after mixing and charging in situ, polymerized at a temperature usually close to 50° C. to 60° C. After the return to ambient temperature, the volume of the explosive charge has reduced and causes quite a considerable clearance between the charge and the body of the munition. This clearance is greater than with a poured-molten charge because the coefficient of thermal expansion of composite explosives is greater than that of poured-molten explosives. Moreover, the munitions must be able to be stored and used in wide temperature ranges, typically of between −50° C. and +70° C. Consequently, the clearance between the explosive charge and the body is likely to vary and may become very great in low temperatures of use and of storage. For gyroscopically stabilized munitions, the presence of a clearance involves the creation of an unbalancing mass which changes the trajectory of the munition and reduces its range. The effect of the unbalancing mass is particularly significant because of the high rotation speed of the munitions. As an example, a rifled mortar projectile with a caliber of 120 millimeters leaves its gun at a rotation speed of approximately 10 000 revolutions per minute. A second problem associated with the physical properties of composite explosives is their low hardness after polymerization in comparison with that of poured-molten explosives. At the beginning of the shot, the powerful axial acceleration causes a contraction of the explosive charge, also meaning a movement of the center of mass and therefore a source of deviation of trajectory. Moreover, for gyroscopically stabilized munitions, the radial acceleration causes a twisting of the explosive charge, this twisting being combined with the contraction of the explosive charge. The result of this is a deformation of the munition, the latter taking a bowed shape likely to change when the accelerations reduce on leaving the gun. This deformation also means a disruption to the flight behavior of the munition.

SUMMARY OF THE INVENTION

The invention notably alleviates all or some of the aforementioned drawbacks by providing a munition, the explosive charge of which remains permanently wedged relative to the body of the munition, that is to say at all temperatures and during all stresses associated with the firing and on the trajectory, while preventing the charge from sticking in the casing. Accordingly, the subject of the invention is a munition comprising a body forming an internal volume, and an explosive charge confined in the internal volume, the explosive charge substantially conforming to the shape of an internal longitudinal surface of the internal volume. This longitudinal surface is substantially of revolution about an axis and comprises protruding portions and/or cavities making it possible to keep the explosive charge wedged in the body.

A notable advantage of the invention is that the wedging of the explosive charge in the body of the munition is carried out statically, with no additional parts. The invention can therefore be adapted to any type of munition without adding significant additional manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given as an example, a description made with respect to appended drawings which represent:

FIG. 1, an exemplary embodiment of a munition according to the invention;

FIG. 2, by a detailed view at a boss according to the invention, the arrangement of an explosive charge relative to a body of the munition when the munition is at a temperature below the polymerization temperature of the explosive charge;

FIG. 3, in a view in section along the sectional plane A-A of FIG. 1, a particular embodiment of a munition according to the invention.

DETAILED DESCRIPTION

The following description is made with reference to a rifled mortar projectile; however, the invention applies to any type of munition comprising an explosive charge, notably missiles, rockets and projectiles.

FIG. 1 represents, in a view in partial section, an exemplary embodiment of a munition according to the invention, in this instance a projectile. The projectile 1 comprises a body 2 of elongate shape and a cup 3 assembled to the body 2 at one of its ends. The cup 3 is for example screwed to the body 2. The portion of the projectile comprising the cup 3 forms the rear portion of the projectile to which the thrust is applied when the projectile leaves. The projectile 1, and in particular its body 2, is substantially of revolution about an axis X. The body 2 is hollow so that the assembly comprising the body 2 and the cup 3 forms an internal volume 5. This internal volume 5 is delimited by an internal surface, called the casing 4. The casing 4 is also substantially of revolution about the axis X, notably for reasons of behavior of the projectile in flight. At the end of the body 2 opposite to the cup 3, a space 6 is arranged for accommodating a device for firing the projectile 1, not shown, capable of triggering the explosion of the projectile 1. The body 2 also comprises a rifled band 7 on its external surface 8. The rifled band 7 makes it possible to impart a rotary movement to the projectile when it leaves. An explosive material 10 is loaded into the internal volume 5. The explosive material 10 is for example composite, that is to say that it comprises explosive molecules coated in a polymer binder. In such a case, the explosive 10 is inserted into the internal volume 5 and is polymerized, typically at a temperature close to 50° C. to 60° C. The explosive charge 10 therefore conforms to the shape of the internal volume 5 at the temperature at which it is polymerized. In particular, a surface 11 corresponding to the longitudinal portion of the casing 4 and a longitudinal surface 12 of the explosive charge 10 match over the whole of their contact surface at the polymerization temperature of the explosive 10. In other words, the explosive charge 10 substantially conforms to the shape of the longitudinal surface 11 of the casing 4. Composite explosives usually have a relatively high coefficient of expansion compared with explosives of the poured-molten type. Consequently, the change of temperature of the explosive charge 10 after its polymerization causes considerable variations of its volume. In particular, the explosive charge 10 shrinks when it cools after polymerization; in other words, it sustains a reduction in its volume after it has cooled. Also, the coefficient of expansion of the explosive charge 10 is typically much greater than the coefficient of expansion of the body 2. Therefore, the body 2 and the explosive charge 10 are not deformed in the same proportions when there is a change of temperature. In particular, when there is a reduction in the ambient temperature, the explosive charge 10 contracts more than the body 2. The result of this, as for the shrinking after polymerization, is the introduction of a peripheral clearance between the explosive charge 10 and the casing 4. This clearance depends notably on the temperature of the body 2 and the explosive charge 10. Typically, munitions must be able to be used and stored in a temperature range of between −50° C. and +70° C. without notable degradation of their performance. This temperature range is called the temperature range of use. Because of the relatively uniform shape of the explosive charge 10, the shrinkage is also uniform, so that the longitudinal surface 11 of the casing 4 and the longitudinal surface 12 of the explosive charge 10 remains substantially matched. In order to alleviate a possible clearance between the body 2 and the explosive charge 10, the longitudinal surface 11 of the casing 4 comprises protruding portions and/or cavities. The longitudinal surface 11 is therefore a surface of revolution give or take the protruding portions and/or the cavities. Each protruding portion and/or each cavity makes it possible to maintain in its vicinity a contact surface between the casing 4 and the explosive charge 10 for any temperature of the explosive charge 10 within a given temperature range. This temperature range is advantageously the temperature range of use of the munition. Consequently, the explosive charge 10 remains wedged in the body 2 of the munition. It remains notably wedged when it shrinks after polymerization and when there is a drop in the ambient temperature. In FIG. 1, the protruding portions and/or the cavities are bosses 14 of a given height. “Boss height” means the thickness of the boss on an axis substantially orthogonal to the axis x. The height of each boss 14 is adapted so as to maintain, at the said boss, a contact surface between the casing 4 and the explosive charge 10 for any temperature within the temperature range of use. The height of the bosses 14 may notably take account of the coefficients of expansion of the body 2 and of the explosive charge 10 and the temperature range of use of the projectile. In the example of FIG. 1, each boss 14 has, in a view in section passing through the axis X, a beveled profile. The profile of the bosses 14 can however be different. It can, for example, form a rounding. The profile of the bosses 14 and, in general, of the protruding portions and/or of the cavities, is adapted so as to keep the explosive charge 10 wedged in the body 2.

FIG. 2 represents in detail the arrangement of the explosive charge 10 at a boss 14 when the projectile 1 is at a temperature below the polymerization temperature of the explosive charge 10. Because of the difference between the coefficients of expansion of the body 2 and of the explosive charge 10, the latter is locally contracted around the boss 14. The contact surface between the body 2 and the explosive charge 10 is therefore reduced, but remains sufficient to immobilize the explosive charge 10 relative to the body 2.

FIG. 3 represents, in a view in section on the sectional plane A-A of FIG. 1, a particular embodiment of a projectile according to the invention. According to this particular embodiment, the protruding portions and/or the cavities, in this case the bosses 14, each form a contact surface which is able to prevent rotation about the axis X of the explosive charge 10 relative to the casing 4. These contact surfaces are naturally formed when the bosses 14 are not circular on the axis X. The bosses 14 then locally form protrusions relative to the longitudinal surface 11 of the casing 4 and constitute exceptions to the shape of revolution on the axis X of the casing 4. As above, the height of each boss 14 and the profile, in the sectional plane A-A, can be adapted so as to prevent the rotation of the explosive charge 10 relative to the body 2. The height of the bosses 14 can then also depend on the radial acceleration when the projectile leaves and on the hardness of the explosive charge 10.

According to one particular embodiment, the longitudinal surface 11 of the casing 4 comprises at least two protruding portions and/or cavities which are not situated at the same level on the axis X, that is to say which have distinct coordinates on the axis X, as shown in FIG. 1. In other words, the protruding portions and/or the cavities may be distributed over the longitudinal surface 11 in the vicinity of two or more distinct planes orthogonal to the axis X, called P₁ and P₂ in FIG. 1. This particular embodiment makes it possible to keep a contact between the explosive charge 10 and the body 2 at several levels on the axis X and therefore to distribute the wedging of the explosive charge 10 better. The number of planes in the vicinity of which protruding portions and/or cavities are formed depends notably on the length of the projectile 1.

According to one particularly advantageous embodiment, the protruding portions and/or the cavities are distributed symmetrically on the axis X, as shown in FIG. 3. This embodiment makes it possible to maintain an axial symmetry on X of the projectile 1, although the shape of the body 2 is not entirely of revolution. This symmetry of the projectile 1 prevents the appearance of an unbalancing mass. Moreover, this embodiment makes it possible to keep the explosive charge 10 centered in the casing 4 of the body 2 for any temperature in the temperature range of use, the explosive charge 10 contracting concentrically about the axis X when there is a drop in temperature. Therefore, the center of gravity of the projectile 1 is not moved with a change of temperature; no unbalancing mass can form despite the introduction of a clearance between the explosive charge 10 and the casing 4. When protruding portions and/or cavities are formed in the vicinity of several planes, they are advantageously offset angularly on the axis X between two adjacent planes, so as to distribute the contact surfaces between the explosive charge 10 and the body 2 over the periphery of the casing 4. Therefore, the deformations of the explosive charge 10 by bending are limited and the latter is better wedged.

According to another particular embodiment, not shown, the longitudinal surface 11 of the casing 4 comprises one or more protruding portions and/or cavities which are circular on the axis X. If the protruding portions and/or cavities are bosses 14, they form conical shoulders. The circular protruding portions and/or circular cavities do not make it possible to prevent a rotation on the axis X of the explosive charge 10 relative to the casing 4. Conversely, they make it possible to preserve a perfect symmetry of revolution on the axis X of the projectile 1. They are therefore particularly well suited to smooth projectiles, i.e. with no rifled band, and, more generally, to munitions the rotation speed of which is zero or relatively low, since there is no longer any inertial force in rotation to be absorbed, for example at the beginning of the shot. If the protruding portions and/or the cavities are not all situated at the same level on the axis X, it is possible to combine this embodiment with the embodiment according to which the protruding portions and/or the cavities each form a contact surface capable of preventing a rotation on the axis X of the explosive charge 10 relative to the casing 4. The casing 4 then comprises on the one hand a protruding portion and/or a circular cavity in the vicinity of a first plane and, on the other hand, protruding portions and/or cavities capable of preventing a rotation of the explosive charge 10 in the vicinity of a second plane.

Still according to a particular embodiment, shown in FIG. 1, the explosive charge 10 is bonded to at least one portion of an internal surface 15 of the cup 3. The explosive charge 10 can also be bonded to a portion of the longitudinal surface 11 of the casing 4 at the rear portion of the projectile. The surface to which the explosive charge 10 is bonded is called the bonding surface 16. When the explosive charge 10 is bonded to the cup 3, it contracts in the direction of the cup 3 and remains in contact with at least one portion of the surface of the protruding portions and/or the cavities. The bonding of the explosive charge 10 ensures that it is in permanent contact with the cup 3. This contact is crucial because it prevents any risk of the projectile 1 exploding when the projectile leaves. Specifically, if the explosive charge 10 is not already in contact with the cup 3 when the projectile leaves, it strikes it violently because of its inertia and of the axial acceleration on the axis X of the body 2 and of the cup 3. The impact may be violent enough to generate vibratory waves initiating the decomposition of the explosive charge 10. Bonding the explosive charge 10 therefore prevents causing the projectile to explode when it leaves.

The munition according to the invention has been described above considering that the protruding portions and/or the cavities are bosses. The bosses and, more generally, the protruding portions are well suited to munitions the body of which is made of cast iron. Specifically, bodies made of cast iron are usually made by molding and the casting cores forming the casing 4 can be easily modified to form such bosses. However, the casing of the body of the munition may also comprise cavities, notably in the form of circular grooves for smooth projectiles. The profile and the depth of the cavities can be determined in a similar manner to the profile and the height of the bosses, that is to say notably as a function of the coefficients of expansion of the body and of the explosive charge, of the temperature range of use, of the radial acceleration when the projectile leaves and of the hardness of the explosive charge. The cavities are, for example, preferable to the bosses when the body 2 of the munition is made of steel. They may also be easily produced by forging or machining. 

1. A munition comprising: a body forming an internal volume, and an explosive charge confined in the internal volume, the explosive charge substantially conforming to the shape of an internal longitudinal surface of the internal volume, the longitudinal surface being substantially of revolution about an axis and comprising protruding portions and/or cavities making it possible to keep the explosive charge wedged in the body.
 2. The munition of claim 1, wherein at least one protruding portion or one cavity forms a contact surface capable of preventing the explosive charge from rotating on the axis relative to the body.
 3. The munition of claim 1, wherein protruding portions and/or cavities are distributed symmetrically along the axis.
 4. The munition of claim 1, wherein at least one protruding portion or one cavity is circular on the axis.
 5. The munition of claim 1, wherein at least two protruding portions and/or cavities are not at the same level on the axis.
 6. The munition of claim 1, wherein a height of the protruding portions and/or a depth of the cavities are a function of a difference between an expansion coefficient of the body and an expansion coefficient of the explosive charge.
 7. The munition of claim 1, wherein the protruding portions are bosses.
 8. The munition of claim 1, further comprising a cup closing one end of the internal volume, the explosive charge being bonded to at least one portion of an internal surface of the cup.
 9. The munition of claim 1, wherein the explosive charge comprises coated explosive molecules in a polymerized binder. 